Kitty Litter / Clay
Soak kitty litter in warm water for a couple of hours, then it for a few minutes with your hands and then drain the water off. Add some spaghum, peat moss or whatever else you would like in it and slap it on the glass. Depending on the brand of kitty litter, it may not take very long until it will be mostly dissolved into a soft solid material and it can be used. Make sure to not let this background dry out! This type of background needs to be wet in order to stay adhered.
As for the type of kitty litter, make sure to get unscented and many people recommend using powdered clay from a pottery store instead of kitty litter due to the higher quality of clay.
Here are some examples of the kitty litter background: